CMP is a combination of chemical reaction and mechanical buffing. A conventional CMP system includes a polishing head with a retaining ring that holds and rotates a wafer (also referred to interchangeably as a substrate) against a rotating polishing pad surface. The polishing pad can be made of cast and sliced polyurethane (or other polymers) with a filler or a urethane coated felt.
During rotation of the wafer against the polishing pad, a slurry of silica (and/or other abrasives) suspended in a mild etchant, such as potassium or ammonium hydroxide, is dispensed onto the polishing pad. The combination of chemical reaction from the slurry and mechanical buffing from the polishing pad removes vertical inconsistencies on the surface of the wafer, thereby forming an extremely flat surface.
During the CMP, a measurement sensor monitors the status of the CMP of the wafer in-situ by measuring the surface of the wafer through a window of the polishing pad. Once the measurement sensor determines that the CMP is complete, i.e., the endpoint has been reached, the CMP can stop.
However, the window can become optically degraded by the CMP and by pad conditioning, thereby degrading in-situ monitoring by the measurement sensor. For example, the pad conditioning can scratch the top surface of the window, which can lead to deflection of the laser from the measurement sensor during in-situ monitoring, therefore negatively affecting the monitoring.
To overcome this deficiency, traditional systems use movable windows that are lowered a predetermined distance during pad conditioning and raised the same distance during CMP. During CMP, traditional systems prefer to position the window at about or slightly below the polishing pad surface. However, since the pad wears down over time due to conditioning and polishing and since the window and pad wear down at different rates, the window will not always be raised to the same position relative to the polishing surface. In fact, it is likely to be raised above the height of the polishing pad surface, especially after many wafer polishings. Consequently, the window will interfere with CMP of the wafer and optically degrade, thereby causing wafer inconsistencies and less accurate measurements by the measurement system.
Accordingly, a new CMP endpoint detection system is needed that overcomes the above-mentioned deficiencies.